The present invention relates to a spectroscopy instrument and in particular to calibration of a drive mechanism for a diffraction grating monochromator of the spectroscopy instrument.
Wavelength selection in a diffraction grating monochromator is achieved by changing the angular position of the grating element. The angular positioning accuracy required depends upon the desired wavelength accuracy, but the positional accuracies required for even modest wavelength accuracies are formidable. For an accuracy of ±0.5 nm such as might be required for an ultraviolet-visible spectrophotometer, the angular accuracy requirement is about ±0.017 degrees. On the other hand for a monochromator intended for use in emission spectroscopy, having for example a wavelength accuracy requirement of ±0.03 nm, the rotational accuracy required is ±0.001 degrees.
Such positional accuracy is difficult to achieve because of errors introduced by the diffraction grating itself and imperfections in its drive mechanism. Thus calibrations have to be performed to provide an accurate conversion between selected angular settings for the drive mechanism of the monochromator and the wavelength of the diffracted light from the monochromator.
The present invention seeks to provide a calibration method that is quick to complete and which provides good accuracy. The invention also seeks to provide a calibrated spectroscopy instrument.